How to defog a windshield

It has to do with temperature and the air's moisture content. On a cold day, any moisture in the air inside your car — from passengers exhaling, snow on your boots, etc. — turns to condensation when it hits air next to the windows that's below a certain temperature, called the dew point. The condensation is what makes your car's windows appear foggy. On a hot, humid day, the opposite happens, when the muggy air outside your car reaches the dew point against your windshield after it's cooled by your AC system.

Whether the fog is on the inside or the outside of your windows, any time you can't see clearly in all directions is a potential driving hazard. So, it's important to know how to make sure your windows are clear — no matter the weather.

Why does my windshield fog up even when it's not cold?

Sometimes you may notice your windshield getting foggy in mild or even warm weather. This can occur when the humidity in your vehicle is higher than outside and your windshield is cooler than the current dew point.

Humidity increases when more moisture is released into the air according to the National Oceanic and Atmospheric Administration (NOAA), so this can happen if you have open drinks or damp items in your car, or even when you breathe.

What causes fogging on the inside vs. the outside of the glass?

Fog on the inside of the glass typically occurs in colder weather. When the warm, moist air inside your car meets the cold surface of your windshield, condensation accumulates on the inside of the windshield.

Outside fogging, on the other hand, usually happens in warm, more humid weather. When you run the air conditioning, it brings the temperature of your windshield down, and when the warm, moist air outside your car meets the cold surface, it condenses on the outside of the glass.

When you're dealing with cold weather outside and you turn on the heat inside your car, the fog typically will start to form on the inside of your car windows and windshield.

If you're trying to defog the windows in cold weather, you want your car to continually take in the drier outside air instead of reusing the more humid air inside the vehicle. If you’re not sure whether your car has recirculation, you’ll want to look for a button on the dashboard that has an arrow going in a circle or a semi-circle. Sometimes, it will feature an icon of a car with this type of arrow inside it.

Four steps to defog windshield in cold weather

The fastest way to defog your windshield and windows, according to Road and Track:

1. Turn the heat on its maximum setting, because hot air can hold more moisture.
2. Turn the AC on, which will pull the moisture from the air as it passes over the cooling coils.
3. Turn off the recirculation button, so colder, dryer air is brought into the car.
4. Crack your windows for a couple minutes, if possible, to help exchange the humid interior air for dryer outside air.

A more comfortable way to defog a windshield when it's cold

If you’re trying to stay warm and snug while driving, turn on the defroster and blow warm air across the windshield to evaporate the accumulating moisture, advises Lifehacker. If your vehicle's ventilation system has a recirculate feature, turn it off. When this feature is on, your car's heat or AC reuses the air inside the car instead of continually pulling in air from outside.

When the temperature and moisture level outside are greater than inside the car, moisture will condense on the exterior of the car glass. Similar to when it's colder outside than the inside of your car, the goal is to change the temperature on the inside of the car to match the outside temperature.

Steps to quickly defogging a hot windshield

In this case, it means warming up the inside. Keep the following tips in mind:

1. Use your windshield wipers. This will help get rid of the condensation until you've balanced out the temperature.
2. Warm up the inside of your car. Turn down the AC to the lowest (least cool) setting to increase the temperature without it becoming too uncomfortable. If this doesn't work, turn the AC off completely, says Lifehacker.
3. Leave recirculation off: As stated before, it's a good idea to turn off your car's recirculation feature to battle foggy windows so the temperature and moisture levels in your car begin to equalize with those outside.

Also, it’s not advised to wipe your windshield or windows with a cloth or fabric. Doing so can lead to streaks which could negatively impact your view in direct sunlight or when driving towards headlights, warns Consumer Reports.

Rainy or humid weather can cause windows to fog up because of the higher-than-usual moisture content in the air. If your windshield is at a much different temperature than outside, then fogging may happen due to the difference in temperature, according to Nissan USA.

The goal in this case is to try to equalize the temperature and moisture level inside your vehicle with the type of weather you’re experiencing. If it’s warm, follow the advice for defogging in warm weather; if it's cold, follow the cold weather advice.

If your vehicle does not have AC or it isn’t operational, you have fewer tools to defog your windows, but you still have options. As with the other tips, your goal is to reduce the humidity in your vehicle and keep the temperature as close to the outside temperature as you can comfortably. Here are a few ideas when you can’t use air conditioning, according to Folsom Lake Honda:

• Crack a window: If the conditions allow for this, you can let fresh air into the car and help the temperature and humidity level equalize more quickly
• You can keep baking soda in your car as a natural dehumidifier
• Use an anti-fog solution, such as a spray
• If you have access to a hairdryer, you can use it on hot as a makeshift defogger.

Most modern vehicles come equipped with multiple tools to help combat windshield fogging. Understanding what each of these tools does can help you respond quickly and accurately in changing weather conditions.

Front defrosters

Front defrosters direct air toward the windshield. When activated, it typically uses both heat and air conditioning to remove moisture from the inside of the glass, according to Auto Zone.

Rear defrosters

Rear defrosters work differently. They use electrical heating elements to gradually warm the glass, evaporate condensation, and melt ice.

What does the windshield defrost button do?

Pressing the defrost button activates airflow aimed at the windshield. Many systems also engage the AC compressor because air conditioning helps dry out the air. This dual-action clears moisture more efficiently from the inside of the glass.

How do rear defoggers work?

Rear window defoggers use electric resistance to heat fine wires embedded in the glass. When you press the rear defrost button, electricity flows through these wires, gently warming the window and clearing moisture or frost.

Dashboard symbols to recognize

The front defrost symbol typically looks like a windshield with three upward-pointing wavy arrows. The rear defrost symbol appears as a rectangle with three vertical wavy lines.

Using both defrosters together

On cold or rainy days, it’s often helpful to use both the front and rear defrosters together. This can help ensure that your windshields remain clear.

Now that you have an idea of what to do when your windshield and windows fog up, let’s look at a few steps to help avoid fog form inside your car. Whether you experience foggy windows often or you’re trying to avoid defogging when you’re in a hurry, these things may help.

Keep your windshield and windows clean

As simple as this sounds, keeping the glass inside your car clean can actually help prevent fogging, explains NAPA. The dirtier your windshield and windows are, the easier it is for condensation to form on them. Clean your windshield and windows from time to time to make it harder for fog to form. NAPA recommends you use a glass cleaner specifically made for cars. If you have tint on your windows, be sure to use a product that’s ammonia-free, as it can eventually cause tint to rip or fade.

Use cat litter to prevent moisture

For times when you get fog on the inside of your windows (generally colder weather), you can use cat litter, that is designed to absorb moisture, to eliminate fog in the first place, explains Reader’s Digest. Look for cat litter that is either non-clumping or made with crystals. Once you have your litter, you can use something like an old pair of socks to put that litter in and place into your car. This should draw out moisture from the inside of your car and prevent the moisture that causes your windshield and windows to fog.

If it’s still a problem, consider an anti-fog spray

When fog in your car continues to be an issue, there are products you can use on the inside of your windshield and windows to prevent. NAPA explains that these products discourage condensation and are easy to apply. You should be able to use a paper towel or cloth with the spray and reapply it after the first application, as needed.

Trying to see through a fogged-up windshield or windows can be a driving hazard, but with these tips, you can help increase your driving safety — no matter what weather you’re driving in.